Halfdan, Halfdene or Healfden Ragnarsson († 877 in Ireland), also called Hvitserk (white robe) because he wore a white tunic, was a son of the legendary Viking leader Ragnar Lothbrok.
Halfdan landed in England in 865 with his brothers Ivar the Boneless and Ubbe and later became the leader of the Great Heathen Army. On January 5, 871, he and his Danes won the Battle of Reading, but lost the Battle of Ashdown four days later. He was ruler of London from 871 to 872, where he struck coins.
Halfdan made raids into Northumbria and Scotland from his winter camp on the Tyne in 874. In 875 Halfdan finally subdued Northumbria. There were already some Vikings living there, primarily from Denmark. In the same year, Halfdan had himself crowned king of Jorvik, the most important city in northern England, which later became known as York.
He is said to have been very unpopular because of his alleged cruelty. In 876 he settled Northumbria, again depriving the Great Heathen Army of one of its most important leaders.
In 877, Halfdan broke a truce between Danes and Anglo-Saxons that had been concluded two years earlier by the West Saxon king Alfred the Great and Halfdan's brother Ubba.
Halfdan fell in 877 fighting Norwegians on Strangford Lough in Ireland. In Jorvik, two new kings filled the power vacuum left by him, either immediately after his death or in 883.